209 research outputs found
Local structure of REFeAsO (RE=La, Pr, Nd, Sm) oxypnictides studied by Fe K-edge EXAFS
Local structure of REOFeAs (RE=La, Pr, Nd, Sm) system has been studied as a
function of chemical pressure varied due to different rare-earth size. Fe
K-edge extended X-ray absorption fine structure (EXAFS) measurements in the
fluorescence mode has permitted to compare systematically the inter-atomic
distances and their mean square relative displacements (MSRD). We find that the
Fe-As bond length and the corresponding MSRD hardly show any change, suggesting
the strongly covalent nature of this bond, while the Fe-Fe and Fe-RE bond
lengths decrease with decreasing rare earth size. The results provide important
information on the atomic correlations that could have direct implication on
the superconductivity and magnetism of REOFeAs system, with the chemical
pressure being a key ingredient
Soft x-rays absorption and high-resolution powder x-ray diffraction study of superconducting CaxLa(1-x)Ba(1.75-x)La(0.25+x)Cu3Oy system
We have studied the electronic structure of unoccupied states measured by O
K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal
structure studied by high resolution powder x-ray diffraction (HRPXRD), of
charge-compensated layered superconducting CaxLa(1-x)Ba(1.75-x)La(0.25+x)Cu3Oy
(0<x<0.4, 6.4<y<7.3) cuprate. A detailed analysis shows that, apart from hole
doping, chemical pressure on the electronically active CuO2 plane due to the
lattice mismatch with the spacer layers greatly influences the superconducting
properties of this system. The results suggest chemical pressure to be the most
plausible parameter to control the maximum critical temperatures (Tcmax) in
different cuprate families at optimum hole density.Comment: 14 pages, 11 figures, accepted for publication in Journal of Physics
and Chemistry of Solid
Sc substitution for Mg in MgB2: effects on Tc and Kohn anomaly
Here we report synthesis and characterization of Mg_{1-x}Sc_{x}B_{2}
(0.12T_{c}>6 K.
We find that the Sc doping moves the chemical potential through the 2D/3D
electronic topological transition (ETT) in the sigma band where the ``shape
resonance" of interband pairing occurs. In the 3D regime beyond the ETT we
observe a hardening of the E_{2g} Raman mode with a significant line-width
narrowing due to suppression of the Kohn anomaly over the range 0<q<2k_{F}.Comment: 8 pages, 4 EPS figures, to be published in Phys. Rev.
Spin correlations in Ca3Co2O6: A polarised-neutron diffraction and Monte Carlo study
We present polarised-neutron diffraction measurements of the Ising-like
spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature
TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW)
structure to a mixture of SDW and commensurate antiferromagnet (CAFM)
structures is observed on cooling. For a rapidly-cooled sample, the majority
phase at low temperature is the SDW, while if the cooling is performed
sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5
and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse
scattering data. We show that both intra- and inter-chain correlations persist
above TN, but are essentially decoupled. Intra-chain correlations resemble the
ferromagnetic Ising model, while inter-chain correlations resemble the
frustrated triangular-lattice antiferromagnet. Using previously-published bulk
property measurements and our neutron diffraction data, we obtain values of the
ferromagnetic and antiferromagnetic exchange interactions and the single-ion
anisotropy.Comment: 10 pages, 7 figure
Intricacies of the Co spin state in SrCoIrO: an x-ray absorption and magnetic circular dichroism study
We report on a combined soft x-ray absorption and magnetic circular dichroism
(XMCD) study at the Co- on the hybrid 3/5 solid state oxide
SrCoIrO with the KNiF structure. Our data
indicate unambiguously a pure high spin state for the Co
(3) ions with a significant unquenched orbital moment
despite the sizeable elongation of the CoO octahedra. Using quantitative
model calculations based on parameters consistent with our spectra, we have
investigated the stability of this high spin state with respect to the
competing low spin and intermediate spin states.Comment: 7 pages, 4 figure
Study of temperature dependent atomic correlations in MgB
We have studied the evolution with temperature of the local as well as the
average crystal structure of MgB using the real-space atomic pair
distribution function (PDF) measured by high resolution neutron powder
diffraction. We have investigated the correlations of the B-B and B-Mg nearest
neighbor pair motion by comparing, in the wide temperature range from T=10 K up
to T=600 K, the mean-square displacements (MSD) of single atoms with the
mean-square relative displacements (MSRD) obtained from the PDF peak
linewidths. The results show that the single atom B and Mg vibrations are
mostly decoupled from each other, with a small predominance of positive (in
phase) correlation factor for both the B-B and B-Mg pairs. The small positive
correlation is almost temperature independent, in contrast with our theoretical
calculations; this can be a direct consequence of the strong decay processes of
the anharmonic phonons
Electronically highly cubic conditions for Ru in alpha-RuCl3
We studied the local Ru 4d electronic structure of alpha-RuCl3 by means of
polarization dependent x-ray absorption spectroscopy at the Ru-L2,3 edges. We
observed a vanishingly small linear dichroism indicating that electronically
the Ru 4d local symmetry is highly cubic. Using full multiplet cluster
calculations we were able to reproduce the spectra excellently and to extract
that the trigonal splitting of the t2g orbitals is -12 meV, i.e.
negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent
with our magnetic circular dichroism measurements, we found that the ratio of
the orbital and spin moments is 2.0, the value expected for a Jeff = 1/2 ground
state. We have thus shown that as far as the Ru 4d local properties are
concerned, alpha-RuCl3 is an ideal candidate for the realization of Kitaev
physics
Bulk and surface electronic properties of SmB6: a hard x-ray photoelectron spectroscopy study
We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy
(HAXPES) measurements on in-situ cleaved and ex-situ polished SmB6 single
crystals. Using the multiplet-structure in the Sm 3d core level spectra, we
determined reliably that the valence of Sm in bulk SmB6 is close to 2.55 at ~5
K. Temperature dependent measurements revealed that the Sm valence gradually
increases to 2.64 at 300 K. From a detailed line shape analysis we can clearly
observe that not only the J=0 but also the J=1 state of the Sm 4f 6
configuration becomes occupied at elevated temperatures. Making use of the
polarization dependence, we were able to identify and extract the Sm 4f
spectral weight of the bulk material. Finally, we revealed that the oxidized or
chemically damaged surface region of the ex-situ polished SmB6 single crystal
is surprisingly thin, about 1 nm only.Comment: 11 pages, 8 figure
Long-range interactions in the effective low energy Hamiltonian of Sr2IrO4: a core level resonant inelastic x-ray scattering study
We have investigated the electronic structure of Sr2IrO4 using core level
resonant inelastic x-ray scattering. The experimental spectra can be well
reproduced using ab initio density functional theory based multiplet ligand
field theory calculations, thereby validating these calculations. We found that
the low-energy, effective Ir t2g orbitals are practically degenerate in energy.
We uncovered that covalency in Sr2IrO4, and generally in iridates, is very
large with substantial oxygen ligand hole character in the Ir t2g Wannier
orbitals. This has far reaching consequences, as not only the onsite
crystal-field energies are determined by the long range crystal-structure, but,
more significantly, magnetic exchange interactions will have long range
distance dependent anisotropies in the spin direction. These findings set
constraints and show pathways for the design of d^5 materials that can host
compass-like magnetic interactions
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